Get a URL object from an HTTP header. bufp and offset must have been retrieved from
a header object previously (such as via TSHttpTxnClientReqGet()). locp is updated
on success to refer to the URL object. Note this is a URL object, from which URL related
information can be extracted.

The value placed in locp is stable only for a single callback, as other callbacks can
change the URL object itself (see TSHttpHdrUrlSet()), not just the data in it. That value is
also valid only if this function return TS_SUCCESS.